1. Field of the Invention
The present invention relates to a hydraulic control device provided with a rotation prioritizing function in a working machine such as a hydraulic excavator.
2. Description of the Related Art
In a hydraulic excavator, as shown in FIG. 3, a boom 3, an arm 4, a bucket 5 and a working (excavating) attachment 9 including a boom cylinder 6, an arm cylinder 7 and a bucket cylinder 8 for driving the boom, the arm and the bucket are mounted to an upper rotating body 2 rotatably around a vertical shaft O mounted on a crawler type lower traveling body 1.
As other hydraulic actuators, left and right traveling motors (not shown) for driving the lower traveling body 1, and a rotation motor 10 for driving and rotating the upper rotating body 2 are provided.
In such a case, for example, a combined operation of arm pulling and rotation for rotating while pulling the arm 4 (extending the arm cylinder 7) after earth and sand are loaded onto a delivery vehicle so as to return to an excavating point is frequently performed.
In such a case, since the arm 4 moves in the free-fall direction, a working pressure of the arm cylinder 7 is lower than working pressure of the rotation motor 10. Therefore, most of pump discharge oil is sent to the arm cylinder 7 so that a flow rate is insufficient in the rotation motor 10. Consequently, rotation speed is remarkably decreased and working efficiency and operability are deteriorated.
There is a technique taking a rotation prioritizing measure in which a rotation prioritizing valve serving as a flow rate control valve is provided on one of the meter-in side and the meter-out side or on both the sides (a description will be given to the case of the meter-in) of a control valve for controlling an action of the arm cylinder 7, and at the time of the combined operation, the working pressure is raised by throttling a meter-in flow rate of the arm cylinder 7 by the rotation prioritizing valve so as to ensure a flow rate supplied to the rotation motor (rotation movement) (refer to Japanese Patent Laid-Open No. 2001-295804).
A description will be given to a circuit configuration thereof with reference to FIG. 4.
It should be noted that in a general hydraulic excavator, all the hydraulic actuators are divided into two groups and the actuators of both the groups are driven by separate hydraulic pumps (first and second hydraulic pumps), while discharge oil of both the pumps are joined together at the time of the combined operation or the like. However, in order to facilitate the description, the circuit configuration in which all the actuators are driven by one hydraulic pump will be described.
For similar reasons, FIG. 4 only shows the boom cylinder 6, the arm cylinder 7 and the rotation motor 10 as the hydraulic actuators.
The discharge oil from a hydraulic pump 11 serving as a common pressure oil supply source for the actuator groups is supplied to the rotation motor 10, the boom cylinder 6 and the arm cylinder 7 through a rotation control valve 12, a boom control valve 13 and an arm control valve 14 (hydraulic pilot switching valves) separately operated by remote controller valves (not shown). T denotes a tank.
On the meter-in side of the arm control valve 14, a rotation prioritizing valve 15 serving as a flow rate control valve of a hydraulic pilot type is provided.
A pilot port 15a of the rotation prioritizing valve 15 is connected to pilot lines 18 and 19 on the both sides of the rotation control valve 12 through a rotation prioritizing pilot line 16 and a shuttle valve 17. At the time of movement of the control valve 12, that is, at the time of rotation movement, the rotation prioritizing valve 15 is switched from an entirely opening position x shown in the figure to a throttling position y.
Thereby, at the time of the combined operation of arm pulling and rotation, the meter-in flow rate of the arm cylinder 7 is throttled and the working pressure of the cylinder 7 is increased so as to ensure an action of the rotation motor 10. That is, a rotation prioritizing function is activated.
In such a state, the flow rate of the arm cylinder is decreased in comparison with the case where the rotation prioritizing function is not activated. However, since a required flow rate is small, especially, in the arm pulling movement, there is no problem in movement of the arm 4.
However, in the circuit shown in the figure, the arm pulling is not distinguished from the arm pushing with the working pressure which is higher than the arm pulling. Therefore, since the rotation prioritizing function is also activated at the time of the combined operation of arm pushing and rotation, the arm pushing movement is slightly deteriorated.
However, in the circuit configuration provided with the rotation prioritizing function, as an adverse effect, when other actuator movement (such as boom lowering movement and bucket movement) is added at the time of the combined operation, that is, in a state that the working pressure of the arm cylinder 7 is increased, the working pressure of the added actuator is relatively low so that the oil is further required and hence the flow rate of the arm cylinder is insufficient. Thereby, since the arm pulling movement is stopped or extremely deteriorated to a state close to the stoppage, movement of the entire attachment is deteriorated so as to cause a problem that the working efficiency is extremely lowered.